This invention relates generally to calibration of radar altimeters within air vehicles, and more specifically, to methods and systems for automatic zero altitude calibration of radar altimeters.
In navigation of an aircraft, altitude accuracy is most important at low altitude levels, particularly during landing operations. Radar altimeters installed in aircraft typically provide AGL (above ground level) altitude. Typically, radar altimeters are calibrated during production to compensate for signal propagation delays specific to a particular aircraft installation. The compensation for aircraft installation signal propagation delays may include one or more of a measurement of the distance between a radar altimeter antenna and the ground when the aircraft is on the ground. Such a measurement can range, for example, from about three feet to about twenty feet. Another compensation is due to a cable length from the radar altimeter to the one or more antennas, typically from about one foot to about thirty feet.
In addition, certain radar altimeter applications utilize a zero feet altitude indication when any of the wheels of the aircraft first touch a surface (i.e., an airport runway, an aircraft carrier deck) during landing. Other radar altimeter applications utilize a zero feet indication when the aircraft is at rest on the ground.
Every new aircraft application of known radar altimeters requires trained personnel, located in the field with the aircraft, to determine and adjust a zero altitude calibration when installing the radar altimeter system (i.e., the radar altimeter and associated antenna(s)) into the aircraft. Any installation changes with regard to the radar altimeter system following the above mentioned zero altitude calibration procedure may require repeating the zero altitude calibration procedure. In addition, normal day to day aircraft operations and maintenance may result in variations and inaccuracies in altitude readings provided by a radar altimeter. These variations and inaccuracies may be particularly noticeable when the aircraft is on the ground. Examples of such installation changes, operations, and maintenance include, but are not limited to, replacement of the tires, changes in cargo load weight, and radar altimeter antenna cable repair and replacement.